Calibrating device



April 3- R. L. DOWNEY 2,315,945

- CALIBRATING DEVICE Filed bee. 26, 1941 Fig. l

OSCILLATOR Zr? \A 221 w gag}? I I min Inventor: Regi na l d L. Downeg,

9 Hmttorneg.

Patented Apr. 6, 1943 CALIBRAT'INGn DEVICE Reginald L. Downey, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application December 26, 1941, Serial No. 424,393

Claims.

My invention relates to calibrating devices, and more particularly to such devices for indicating coincidence between the unknown frequency of a wave and a standard frequency.

One common form of such a calibrating device includes a standard frequency oscillator, means for heterodyning the output of the standard frequency oscillator with the wave of unknown frequency, and means for producing an indication of the beat frequencies produced by the heterodyne operation. Confusion and incorrect indications often result with such apparatus because of the complexity of the heterodyne products, particularly those of the higher frequencies. It is, accordingly, an object of my invention to reduce the possibility of such confusion and incorrect operation of such a system.

It is a further object of my invention to provide such a calibrating device of simplified and improved form, which is easily operated in minimum time with minimum chance of error.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 schematically illustrates a circuit arrangement embodying my invention, and Fig. 2 is a perspective view of a visual indicating device which may be used in the embodiment of Fig. 1.

In Fig. 1 I have represented a variable frequency oscillator 10, whose dial is to be calibrated, in terms of its output frequency. This oscillation may be considered to transmit oscillations to an electron discharge device II, in which heterodyne action takes place with oscillations supplied by a standard frequency oscillator including a piezoelectric vibratordevice l2 and an electron discharge device [3. Oscillations resulting from the heterodyne action are impressed on a cathode ray visual indicating device M arranged to indicate the frequency relation between waves from the oscillator l0 and from the device l2. The oscillator. Ill has a control knob by which the frequency of its output may be adjusted between certain limits. When the control knob 15 is adjusted to a certain setting, which may be represented by a particular marking l3 on the attached dial, it is intended that the output of the oscillator l0 shall be equal in frequency with the frequency of operation of the device I2.

Means are provided whereby the frequency of the output of the oscillator I0 may be adjusted, without moving the control knob I5, so'as to make this frequency coincide with the standard frequency of the output of the device l2. It is usual in variable frequency oscillators, such as the oscillator Hi, to adjust frequency by varying the capacity of a tuning condenser, such as the condenser I! in the oscillatory circuit of the oscillator Ill. The means for adjusting the frequency of the output of oscillator l0 without adjusting the control knob l5 may conveniently include a second, or trimming, condenser 18', connected in shunt to the main tuning condenser l1, and adjustable by a second control knob 19.

In the standard frequency oscillator, with the output of which the output of the oscillator I0 is heterodyned, the piezoelectric vibrator device I2 is connected in shunt to a resistance 20 between the control electrode 2! and cathode 22 of the discharge device l3. A tuned circuit 23 is connected in series with a bypassing condenser 24 between the anode 25 and cathode 22 of the discharge device I3 to form a tuned output circuit therefor. Operating current for the device I3 is supplied from a source of potential 26. The negative terminal of source 26 is connected to the cathode 22 and to ground, and the positive terminal is connected through a resistance 21 and through the tuned circuit 23 to the anode 25. The oscillator operates in normal fashion, the piezoelectric vibrator device l2 being maintained in oscillation by feedback from the tuned output circuit 23 through interelectrode capacity between the anode 25 and control electrode 2|. The resistance 20 provides a path for the flow of grid current between the control electrode 2! and cathode 22 and maintains a suitable operating bias on the control electrode 2|;

The output of the oscillator including the device l2 and discharge device I3 is impressed through a coupling condenser 28 upon the control electrode 29 of the discharge device II, and the output of the oscillator I0 is impressed on the same control electrode 29 through a coupling condenser 30. The cathode 3| of the discharge device H is connected to ground, and to control electrode 29 through a resistance 32, which resistance provides a path for the flow of grid current. A variable condenser 38 is connected between the anode 33 and cathode 3| of the discharge device II, and is adjusted to such a value of capacity that the anode 33 of the device ll varies in voltage primarily by reason of heterodyne action between the outputs of the two oscillators. That is, the condenser 38 provides a more or less complete short circuit between the anode 33 and cathode 3| for waves of the frequency of the waves from the oscillator I and from the standard frequency oscillator. The condenser 38 has substantial reactance at considerably lower frequencies, such as the frequency of waves produced by heterodyne action between the outputs of the two oscillators.

Operating current for the discharge device H is supplied from the positive terminal of the source 26 of potential through a resistance 34 to the anode 33, and this current flows back through the cathode 3| and ground to .the negative terminal of source. 25.

The discharge device II is of the duplex type, in which two sections are provided, one forming a triode comprising the cathode 3|, control electrode 29 and anode 33, and the other section comprising the cathode 3| and a pair of anodes 35 forming a diode. This diode 35-3i is used to rectify the heterodyne waves appearing on the anode 33 to produce a continuous potential varying at a frequency equal to the difference between the frequencies of the output of the oscillator I0 and the piezoelectric vibrator device I2. A resistance 36 is connected between the anode 35 and the cathode 3|, and a coupling condenser 31, connected between the anode 35 and the anode 33, transmits heterodyne waves from the anode 33 to the anode 35 to be there rectified. The coupling condenser 31 and the resistance 36 constitute a high pass filter. The rectified voltages from the diode 35--3I appear across the resistance 36 from which they are transmitted to the cathode ray visual indication device I4.

This cathode ray device I4 is illustrated in detail in Fig. 2, in which a pin terminal base 58 supports a glass envelope 39 within which the operating portions of the device are enclosed. A portion of the glass envelope 39 is broken away in order to show more clearly the internal construction of the device. In the lower portion of the envelope 39, formed integrally therewith, is a press 40 through which connections pas from the pin terminals of the base 58 to the electrode elements of the device within the envelope 39. These connections also provide support for the electrodes. Just above the press 40 a mica disc 4| is supported on two connections 42 and 43 at the opposite ends of the press 4|], and in turn the mica disc 45 supports pillars 44 and 45 upon which a second mica disc 45 is mounted part way between the mica disc 4| and the end of the envelope 4! opposite to the base 58.

Between the mica discs 4| and 46 a triode electron discharge section is formed including a cylindrical metal anode 48, a helical control electrode 49 and an indirectly heated coated cathode 50. A suitable connection is provided from each of these three electrodes and from the terminals of the heater for the cathode 54 to the terminals of the base 58.

The upper end of the cathode 50 extends through and above the mica disc 46, to a point within a dish-shaped electrode supported above the disc 46 on columns 44 and 45. That portion of the cathode 50 near the mica disc 46 is uncoated so as not to emit electrons. The upper end 52 of the cathode 50 within the electrode 5| is coated for the emission of electrons. The interior surface of the dish-shaped electrode 5| is coated with material adapted to produce fluorescence upon bombardment by electrons from the coated end 52 of the cathode 5H.

Such fluorescence results when a suitable positive potential is applied to the connection 42 or 43 with respect to the connection to the cathode 50, and it appears on those portions of the internal surface of the electrode 5| which are bombarded by electrons from the coated portion 52 of the cathode 53. The resulting fluorescence may be viewed through the transparent end 41 of the envelope 39.

A control electrode 53, in the shape of small 'rod or narrow flat strip is placed parallel to the upper end 52 of the cathode 50 between the cathode and the dish-shaped electrode 5| for the purpose of electrostatically shielding a variable portion of the internal surface of the electrode 5| from bombardment by electrons from the coated portion 52 of the cathode 50. The potential of the electrode 53 with respect tn the coated portion 52 of the cathode 50 is determined by a connection to the anode 48 from the electrode 53 through a supporting post 54 mounted between the mica discs 4| and 4B and attached to the anode 48.

Referring again to the device I4 as illustrated in Fig. 1, the connections therefor may be explained to show how visual indications are produced by the device I4 in response to heterodyne action between the output of the oscillator I0 and the standard frequency vibrator device I2. Corresponding parts of the device I4 in Fig. 1 have been given the same numerals as like parts in Fig. 2. A variable tap 68 on resistance 35 is connected through a resistance 6| to the control electrode 49 of the device I4, which electrode is connected through a bypassing condenser 82 to the cathode 50. The resistance 6| cooperating with the condenser 62 forms a low pass filter, whose adjustment and method of operation is described hereinafter. Operating current for the device I4 is supplied from the positive terminal of the source 26 of potential through conductor 63 to the anode 5|, and from thence through resistance 64 to the anode 48.

In operation, the voltage of the control electrode 49 with respect to the cathode 50 determines the amount of area on the internal surface of the dish-shaped anode 4| which is not bombarded by electrons, and which correspondingly does not fiuoresce to produce light. This area is generally in the shape of a sector of a circle formed by two radii extending from the end portion 52 of the cathode 58. If the control electrode 49 is at a high negative potential with respect to the oathode 50, so that discharge current does not flow from the cathode 58 to the anode 48; the anode 48, and consequently the control electrode 53, becomes highly positive with respect to the end portion 52 of the cathode 50. The result is that electrons from the end portion 52 of the cathode 50 not only bombard all portions of the internal surface of the anode 5|, but electrons from certain portions of the cathode 53 to the left of the control electrode 53'pass in a curved line of flight to bombard the internal surface of the anode 51 at a considerable distance to the right of the control electrode 53, while electrons emitted from certain portions of the cathode 5D to the right of the control electrode 53 follow an oppositely curved path to bombard portions of the internal surface of the anode 5| to the left of the control electrode 53. Such curved paths are produced because of the fact that the control electrode 53 strongly attracts electrons emitted from the end portion 52 of the cathode 5|). There is consequently produced a fluorescent pattern on the internal surface of the anode 5| in which the entire surface fluoresces to produce light but in which a portion of the internal surface of the anode 5| adjacent the control electrode 53 fluoresces more strongly than the remaining portions, thereby exhibiting an overlapping effect. The edges of this region of overlapping are sharply defined because the control electrode 53 is maintained positive with respect to the end portion 52 of the cathode 50.

When the control electrode 49 is less negative with respect to the cathode 50, so that some discharge current flows from the cathode 50 to the anode 48, a potential drop is produced through the resistance 64 by this discharge current with the consequence that the anode 48 is reduced in potential, and the control electrode 53 is made less positive with respect to the end portion 52 of the cathode 50. With this reduction in positive potential of the control electrode 53, the angle of overlap of fluorescence on the internal surface of the anode 5| decreases, the edges of this overlap still remaining sharply defined because the control electrode 53 is still at a positive potential with respect to the end portion 52.

If the control electrode 49 assumes a minimum negative potential, or even assumes some positive potential, the potential of the anode 48 is reduced to a small positive potential and the potential of the control electrode 53 is correspondingly reduced, however, always remaining positive with respect to the end portion 52 of the cathode 50. During this reduction in potential of the anode 48 a point is reached at which the overlap of fluorescence on the internal surface of the anode 5| no longer exists. Upon further decrease in potential of the anode 48 an increasingly large part of the internal surface of the anode 5| becomes dark. At all times the edges of this area are sharply defined by reason of the fact that the control electrode 53 is always positive with respect to the end portion 52 of the cathode 55.

In operation, the apparatus of Fig. 1, as explained previously, produces heterodyne voltage across the resistance 36 by interaction between the output waves of the oscillator l and the standard frequency vibrator device |2. If the frequency of the Waves from the oscillator l0 differs from the frequency of operation of the vibrator device l2 by only a small frequency, the heterodyne wave across the resistance 36 is of this small frequency, and this heterodyne wave applied between control electrode 49 and the cathode 50 causes the control electrode 53 to vary in voltage at the same frequency, thereby to change the shadow angle produced by the fluorescence on the internal surface of the anode 5| at such frequency.

It is well known that fluorescent materials produce light for an appreciable time after being excited into fluorescence in some manner, as, by bombardment by electrons. When the frequency of the heterodyne wave applied to the control electrode 49 is low, this persistence of fluorescence on the internal surface of the dish-shaped anode 5| is of little consequence, and the edges of the shadow angle remain sharply defined. As the difference in frequency between the frequency of the waves from the oscillator H1 and from the vibrator device l2 increases, the frequency of the heterodyne wave applied to the control electrode 49 increases, and a flicker is noticeable. Eventually, upon further increase in frequency of the heterodyne wave, the flicker disappears due to the effect, of persistence of fluorescence, and a somewhat blurred light portion is visible. The edges of this light portion, however, remain distinct.

By the use of the low pass filter including the resistance BI and the condenser 62 heterodyne waves across the resistance 36 whose frequency is higher than any desired frequency are prevented from appearing in any substantial amount on the control electrode 40. By the diversion of such heterodyne waves, the response of the visual indication device 4| is made to suit the individual tastes of each user.

In a particular case the fluorescent material on the internal surface of the anode 5| of the visual indication device l4 may, for example, be of such character that a wave applied to the control electrode 49 whose frequency is higher than about 30 or 40 cycles causes little flicker, the motion of the shadow angle being more rapid at such input frequency than the period of fluorescent decay of the fluorescent material. It may then be desired to construct the low pass filter so that the reactance of condenser 62 is substantially equal to the resistance 6| at this frequency above which substantial flicker is not noticeable. If this frequency be, for example, 30 cycles, the resistance 6| might suitably be /2 megohm and the condenser 52 might be about 10,000 micromicrofarads.

The arrangement as just described above is useful for calibrating the frequency of the oscillator 0 in terms of the frequency of operation of the vibrator device I2, where the frequency change in the output of the oscillator l0 caused by the control knob l9 need not be very large with respect to the highest frequency indicated by the visual indicator M, which in the case described is a frequency somewhat higher than 30 cycles. The arrangement is suitable, for example, if the frequency change caused by adjustment of the control knob I9 does not, for example, exceed about 1 kilocycle. This small frequency change produced by the control knob H3 is sufficient if the frequency of output of the oscillator l0 when the control knob I5 is set at the mark I6 is not more than a few megacycles.

Cases may arise, however, where the frequency of the output of oscillator H) is of the order of 10 to 50 megacycles, or even higher. In such cases the frequency variation of the output of oscillator I!) caused by adjustment of the control knob I9 when the control knob I5 is set at the mark I6 which must be much larger than 1 kilocycle.

It is in general feasible to make the relationship between the variable condensers I1 and I8 such that adjustment of the control knob 9 is capable of producing about a five per cent change in frequency of the output of oscillator II! when the control knob I5 is set at the mark l6. If the nominal frequency represented by the mark |6 be, for example, 50 megacycles, it is then desirable that the change in frequency produced by adjustment of the control knob H! be an amount in the order of 2 or 3 megacycles. With the low pass filter including the resistor 6| and condenser 62 adjusted as described previously, an angular adjustment of the control knob I9 amounting to about one minute of angle is sufficient to produce a frequency change in the output of the oscillator I0 sufiicient to cause the frequency of the heterodyne wave across the resistance 36 to vary all the way from a frequency higher than the highest frequency to which the device I4 is responsive to zero frequency and back again to a frequency higher than the high frequency to which the device I4 is responsive. It can therefore be seen that under such conditions of adjustment of the low pass filter including resistance Bl andcondenser 62, adjustment of the control knob IS in accordance with indications of the visual indicator device [4 would be extremely difficult. It is desirable in such cases to make the cutoff frequency of thelow pass filter higher than in cases where the nominal frequency represented by the. mark [5 is of a lower value.

In the example given, the output frequency of the oscillator l when the control knob I is set at the mark [6 may, for example, be '50 megacycles, and adjustment of the control knob'l9 is then effective to produce a change of about 2 megacycles in the output frequency. It'may be desired to provide that the visual indicating device l4 shall produce a progressively varying indication as the frequency of the outputof oscillator it] is varied over a range of frequencies about 100 kilocycles in width. The frequency band, over which it is desirable that the Visual indicating device l4 shall respond, depends on the degree of accuracy to which it is desired that the oscillator l0 may be adjusted.

If the oscillator lil'is capabl of very accurate adjustment by adjustment of the control knob l5, highly accurate calibration is necessary, and the cutoff frequency of the low pass filter should be made smaller, with corresponding greater difficulty in the calibration. If then it be, for example, desired that the device M shall produce indications over a band 100 kilocycles in width as produced by adjustment of the control knob IS, the reactance of the condenser 62 may be made equal to the resistance 6i at about kilocycles. The reason for this choice may be seen later, when the operation of the device under such conditions is explained. Suitable values for the resistance GI and condenser 62 under such conditions may, for example, be respectively 6,000 ohms and 10,000 micromicrofarads. In certain cases the visual indicating device l4 may be separated from the remainder of the apparatus by a short distance, such that the control electrode 49 is connected to the resistance 6| by a, shielded cable, or the like. Such cable has appreciable capacity to ground and consequently to the cathod 56, which capacity may constitute the condenser 62. Such cable capacity may, for example, be of the order of 20 to 50 micromicrofarads, and a suitable value for resistance 61 may be chosen to cooperate with this capacity in many cases.

In the operation of the'device so adjusted for calibrating the oscillator H) with a high frequency output, with the control knob 15 set at the mark Hi the control knob I9 is adjusted and, assuming that the control knob I9 is capable of adjusting the output frequency of oscillator Ill through the frequency of operation of the vibrator device l2, the shadow angle of the visual indicating device 14 remains open until the output frequency of oscillator l0 approaches to within a certain frequency difference from thefrequency of operation of the device it, which in the example described above would be 50 kilocycles. With such frequency difierence between the output of oscillator l0 and discharge device 53, the heterodyne wave across the resistance 36 has a frequency equal to such frequency difference, and the low pass filter including resistance 6! and condenser 62 strongly attenuates the heterodyne wave so that a, small portion appears on the control electrode 49. Since this wave is of high frequency, the fluorescent pattern on the internal surface'of the anode 5| is increased in area, the shadow angle" decreasing in size in accordance with the high frequency voltage. Since, as explained before,tthis'high frequency heterodyne waveclosesthe shadow angle and allows it to open again at such a highrate of speed that the fluorescent effect persists from cycle to cycle, it appears that.

resistance 35 decreases so that more of the Wavesv are applied to the control electrode 49 through the low pass. filter, and correspondingly the shadow angle of the device 14 decreases further, until a frequency difference is reached at which the persistence effect of the fluorescent material on the anode -5l caused by this low frequency difference to be visible. filter. including the resistance 5| and condenser 62 produces practically no attenuation, so' that the shadow angle of the device l4, althoughflickering, appears substantially in its most closed position, which may be overlapped as explained previously. The frequency difference at which this effect is observed may be sufficiently small as to be within the limits of accuracy to which the frequency of oscillator It! may be adjusted by adjustment of member is.

If it is. possible to adjust the control knob l9 with greater accuracy, and provided such accuracy be desired, further adjustment of the frequency of the output of oscillator H) in the same direction decreases the frequency of the heterodyne wave across the resistance 36, and slows down the flicker of the shadow angle of the device It to a lower and lower rate ultimately approaching zero as indicated by opening of the shadow effects. Any desired accuracy of adjustment of the oscillator It) may therefore be attained by the use of the low pass filter including the resistance GI and condenser 62 in conjunction with the visual indicating. fluorescent device Hi. this high accuracy, particularly at low frequencies where the movement of the shadow angle of the device H3 is observable, is the particular construction of the visual indicating device shown in Fig. 2, where the use of the control electrode 53 results in a sharply defined edge of the electron beam bombarding the luminescent material on the inside surface of the anode 5 l.

I have so far described the operation of the apparatus of Fig. 1 assuming that the heterodyne waves produced on the anode 33 of the'device- II are substantially unattenuated in being transferred to the anode 35. The apparatus may be so adjusted by utilizing a condenser 31 of sufficient size.

In some cases it may be preferred to prevent the visual indicating device It from flickering at very low frequencies, which may disturb the operator engaged in other duties. It is generally unnecessar to adjust the knob i9 with such accuracy that the flicker of the device It reaches a zero rate, and it is usually necessary only to reduce the rate of this flicker to a very'small percentage of the frequency of the oscillator). The size of the condenser 3'! may be reduced to a value such that frequencies below this limit beyond which adjustment of the knob l9 isunnecessary are not transmitted to the anode 35. Under such conditions of adjustmennthe device I4 produces no flicker within a small frequency allows the flicker:

At such-low frequencies the low pass A contributing factor in making possible band over which any adjustment of the knob I9 is satisfactory.

Although I have described the apparatus including the low pass filter BI, 62 and the visual indicating device M as being used for indicating the frequency difference between the outputs of two oscillators, it is within the scope of my invention to utilize the combination of such a low pass filter and visual indicating device for determining when the frequency of any wave approaches Zero within any desired proximity. The combination of the high pass filter including condenser 3! and resistance 36, the low pass filter including resistance BI and condenser 62, and the visual indicating device in the manner described is especially useful since the opening of the shadow angle of the visual indicating device gradually increases in a natural and expected manner as the frequency of the applied wave decreases, and the closure of the shadow angle is maximum when the frequency of the applied wave is minimum.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a visual indicating device having a fluorescent surface and means including a control electrode for producing fluorescence over a portion of the area of said surface variable in response to voltage on said controlelectrode, a source of alternating voltage whose frequency may vary over a range of frequencies over which indications of frequency are desired, and low pass filter means for applying said alternating voltage to said control electrode attenuated in progressively increasing amounts as the frequency of such voltage increases, the attenuation of said voltage being substantially a maximum at an extreme frequency of said range and being progressively smaller at frequencies removed from said extreme frequency, whereby minimum visual indication is produced by said device when the frequency of said voltage is high and progressively greater indication is produced as the frequency of said voltage approaches zero.

2. In combination, a visual indicating device having means for producing a light pattern of area variable in response to a control voltage applied to said device, a source of alternating voltage whose frequency may vary over a range of frequencies over which indications of frequency are desired, and low pass filter means for applying said alternating voltage to said device attenuated by progressively increasing amount as the frequency of such voltage increases over said range of frequencies, whereby minimum visual indication is produced by said device when the frequency of said voltage is high and progressively greater indication is produced by said device as the frequency of said voltage approaches zero.

3. In combination, a visual indicating device having means for producing a light pattern of area variable in response to a control voltage applied to said device, high pas filter means and low pass filter means connected in cascade for applying a control voltage to said device attenuated in progressively decreasing amounts as the frequency of such voltag decrease toward a predetermined value and attenuated in progressively increasing amounts as the frequency of such voltage decreases further approaching a zero value, whereby minimum visual indication is produced by said device when the frequency of said voltage is high and progressively greater indication is produced by said device as the frequency of said voltage decreases toward said predetermined value, the visual indication produced by said device decreasing as the frequency of said voltage decreases further.

4. In combination, a visual indicating device having a fluorescent surface, electron emitting means for bombarding said fluorescent surface uniformly with electrons to produce fluorescence uniformly thereover, a control electrode positioned between said electron emitting means and said surface, means for maintaining said control electrode at a positive potential with respect to said electron emitting means whereby electrons traveling from said emitting means to said surface near said control electrode are captured thereby to produce a sharply defined edge to the area of bombardment of. said surface by said electrons, said control electrode being effective upon the application of a postive control voltage thereto to vary the area of electron bombardment of said surface and the area of consequent fluorescence, and high pass filter means, and low pass filter means connected in cascade for supplying a control voltage to said control electrode attenuated in progressively decreasing amounts as the frequency of such voltage decreases toward a predetermined value and attenuated in progressively increasing amounts as the frequency of such voltage decreases further, whereby minimum visual indication is produced by said device when the frequency of said voltage is high and progressively greater indication is produced by said device as the frequency of said voltage decreases toward said predetermined value, the visual indication produced by said device decreasing as the frequency of said voltage decreases further.

5. In combination, a source of voltage of known frequency, a source of voltage of variable frequency to be calibrated in terms of the frequency of voltage from said first source, means for producing heterodyne action between the voltages from said two sources to produce a resultant wave whose frequency is equal to the difference between the frequencies of the voltages from said two sources, a visual indicating device having a control electrode and means for producing a light pattern.WhOSe area varies in response to voltage on said electrode, and high pass filter means and low pass filter means connected in cascade for applying said waves to said control electrode at tenuated in progressively decreasing amounts as the frequency of said wave decreases toward a predetermined value and attenuated in progressively increasing amounts as the frequency of such voltage decreases further, whereby minimum visual indication is produced by said device when the frequency of said wave is high and progressively greater indication is produced by said device as the frequency of said Wave decreases toward said predetermined value, the visual indication produced by said device decreasing as the frequency of said voltage decreases further thereby to facilitate adjusting the frequency of voltagefrom said second source to be equal to the frequency of voltage from said first source.

REGINALD L. DOWN EY. 

